1. Field
The present invention relates generally to a satellite positioning system (SPS), and more particularly, to assisting a mobile station to determine its position and time using SPS orbit information.
2. Background Information
A satellite positioning system (SPS) receiver normally determines its position by computing times of arrival of signals from multiple satellites. These satellites transmit, as part of their messages, both satellite positioning data and satellite clock timing data. The satellite positions, velocity and clock timing typically are represented by almanac and ephemeris data. The ephemeris data refers to the content of subframes 1, 2 and 3 of the messages transmitted from a satellite. The ephemeris provides an extremely accurate estimate (˜1 meter error) orbit (satellite positions, clock and clock bias). However, the typical process of searching for and acquiring satellite signals, reading the ephemeris data transmitted by the satellites, and computing the location of the receiver from this data is time consuming and requires moderately strong signal levels.
For example, Global Positioning System (GPS) devices determine position based on the measurement of the times of arrival at a GPS receiver of the GPS signals broadcast from orbiting satellites. As stated, one disadvantage of such a system is the relatively long time needed to perform standalone signal acquisition. Satellite signals cannot be tracked until they have first been located by searching in a two-dimensional search “space”, whose dimensions are code-phase delay and observed Doppler frequency shift. The process of an SPS receiver searching for, acquiring, and demodulating satellite signals is sometimes referred to as a “standalone” mode of operation, which can be contrasted with an “assisted” mode of operation.
In order to reduce the delay associated with a standalone mode of operation, information may be provided to aid an SPS or GPS receiver in acquiring a particular signal. Such assistance information permits a receiver to narrow the search space that must be searched in order to locate a signal, by providing bounds on the code and frequency uncertainty. A system that employs a GPS receiver augmented with GPS assistance data is commonly referred to as an “assisted global positioning system” (AGPS).
One example of an AGPS system includes a wireless mobile station (MS) (such as a cellular telephone) having, or in communication with, a GPS receiver, the MS in communication with one or more base stations (BSs), also referred to as base transceiver substations (BTSs) or node Bs, of a wireless communication network, which in turn communicate with one or more location assistance servers, sometimes referred to as Position Determination Entities (PDEs) or Serving Mobile Location Centers (SMLCs), depending upon the communication air interface protocol. Another example of an AGPS system includes a MS or laptop, having, or in communication with, a GPS receiver, the MS or laptop capable of communication with a communication network, such as but not limited to, the Internet, through which the device communicates with a location assistance server.
The location assistance server derives GPS assistance information from one or more GPS reference receivers (wide area of global reference network). The location assistance server also has access to a means of determining the approximate mobile station position. The location assistance server maintains a GPS database that may include reference time, satellite orbit almanac and ephemeris information, ionosphere information, and satellite working condition (“health”) information. The location assistance server also computes the assistance information customized for the approximate mobile station position.
The position of a MS in an AGPS system can be determined at the MS (sometimes referred to as MS-based positioning mode) with assistance from a location assistance server. During MS-based positioning mode, when a GPS engine requires updated aiding data such as ephemeris data, almanac data regarding the location of satellites or base stations, timing information for the base stations and/or satellites, or seed position (such as, but not limited to that determined by advanced forward link trilateration (AFLT)), and so on, the next position fix will result in the mobile station contacting the communication network for data, thereby taxing the network and using power resources of the MS. The position of a MS in an AGPS system can alternatively be determined at the location assistance server and transmitted back to the MS using information acquired by the MS (sometimes referred to as MS-assisted positioning mode). SPS Satellite orbits can be modeled as modified elliptical orbits with correction terms to account for various perturbations. The relative short-term ephemeris data provides a very accurate representation of the orbit of the satellite. For example, bit 17 in word 10 of GPS subframe 2 is a “fit interval” flag which indicates the curve fit interval used by the GPS control segment in determining the ephemeris parameters with “0” indicating a 4-hour fit and “1” indicating a “greater than 4 hours” fit. Furthermore, the extended navigation mode of the Block II/IIA GPS satellites guarantees the transmission of correct ephemeris parameters for 14 days to support short-term extended operation. During normal operation, the control segment provides daily uploads of the navigation (orbital) data to each satellite to support a positioning accuracy of 16 meters spherical error probable (SEP).
The position of a MS in an AGPS system can also be determined at the MS using ephemeris data directly received from satellites. The ephemeris data, during its period of validity (e.g., a 4-hour epoch), is more accurate than almanac data and predicted orbit data. Predicted orbit data is an estimate of satellite position, velocity and timing based on an orbit solution predicted by a system other than the real time satellite positioning system (e.g. GPS Control Segment). However, the broadcast ephemeris data may not be available to a mobile station all the time due to lack of line of sight, shadowing, poor signal conditions or other reception problems that prevents the MS from demodulating satellite broadcasts and, when available, will still require time to demodulate.
A system and method is needed to enable an SPS receiver to utilize available orbit data to produce accurate positions and timing even when current real time orbit and clock bias information is not available (either from broadcast data or from location assistance server data).